Remove the moldings, trim, and anything you don't want oversprayed or with tape lines.
Wash the car with strong de-grease dishwasher detergent.
Wash it again,
Wash it again.
Get a bucket of soapy water, and a pack of wet dry sand paper in 320 grit.
Now, with a good rubber block (I suggest a 4" and a 9") wet block sand the car using sweeping ever changing angles and direction with plenty of water to keep from digging any deep scrathes in the car.
After doing the whole car, jump up to 400 grit and do it all again.
You might want to start with 220 or 180 on the bumpers if there is any flaking, cracking, or spider webbing and basically block them all the way down to the rubber. When you see the spider web lines go away, sand just a little more. You can get special SEM rubber bumper primer in a can from the auto paint store to blow those back in, and block to make sure the spider webs are gone.
This method is SLOW, and time consuming. However, using such mild paper will keep a firstimer out of trouble and allow you to really prep and block the surface to perfection.
This is for a car with simply a dead paint job.
If you already have multiple paint jobs on the car, you will need more aggressive methods of stripping it down using stronger grit paper and machines to fast sand, then little by little step to smoother and smoother grit to finish it out.